Electrochemical fuel cells electrochemically convert a fuel and an oxidant to produce electric power. A fuel cell includes an anode, a cathode and an electrolyte. Fuel- and oxidant-containing reactant streams are supplied to the anode and cathode of the fuel cell, respectively, in order for it to produce electric power.
In electrochemical fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. In solid polymer fuel cells an ion exchange membrane electrolyte facilitates the migration of protons from the anode to the cathode. In addition to conducting protons, the membrane isolates the hydrogen-containing fuel stream from the oxidant stream. At the cathode, protons that have crossed the membrane react with oxygen to form water as the reaction product.
The anode and cathode reactions in such fuel cells are shown in equations (1) and (2) below: EQU anode reaction H.sub.2 .fwdarw.2H.sup.+ +2e (1) EQU cathode reaction 1/20.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 O(2)
Two or more fuel cells may be electrically connected typically in series, or sometimes in parallel, in fuel cell stacks to increase the overall power output of the assembly. The stack typically includes feed manifolds or inlets for directing the fuel (a hydrogen-containing gas stream) and the oxidant (an oxygen-containing gas stream) to the anode and cathode of the individual fuel cells. The stack also generally includes exhaust manifolds or outlets for expelling the fuel and oxidant streams, each carrying product water.
Fuel cell power plants are of particular significance in the submarine industry, as they offer significant advantages relative to conventional diesel-electric and nuclear power plants often used in submarines. Fuel cell power plants offer high energy conversion efficiency and operate quietly, thereby limiting the opportunity for detection of the acoustic signature of the vessel. Air independent submarine propulsion systems, with on-board storage of fuel and oxidant, avoid the need for snorting periods (drawing air from above the ocean surface) during a mission, reducing the optical and radar signature of the vessel. The fuel cell propulsion system requires less on-board oxygen and fuel storage for a given mission than other air independent propulsion systems because of the high efficiency of the fuel cell as an energy conversion device. Fuel cell powered submarines therefore offer the advantage of long submerged mission durations. Also, relative to nuclear power plants, fuel cell power plants have fewer human health and safety concerns, particularly as to fuel storage and handling.
In submarine applications, one or more fuel cell stacks may be connected in series or, more commonly, in parallel with a conventional battery bank. Individual fuel cells and stacks are electrically connected to meet the desired voltage and current requirements of the system. The fuel cell may be used as the primary source of power for submarine propulsion and for other loads on board, and for charging the batteries. The battery may be used to augment the fuel cell stack power output when power in excess of the maximum fuel cell output is required.
In fuel cell power plants, such as those used to power submarines, the fuel cell stack may be supplied with an oxidant stream composed of substantially pure oxygen or oxygen in a carrier gas such as nitrogen. The oxygen may be stored on-board the submarine as liquid oxygen ("LOX"), or may be generated on-board by decomposition of an oxygen source such as hydrogen peroxide. In some instances, the fuel cell power plant may be adapted to use air as the oxidant stream during surface operations, thereby reserving the stored oxygen supply for operation during submersion.
The fuel stream in such systems is often substantially pure hydrogen obtained by purification of a hydrogen-containing reformate stream generated by the on-board catalytic steam reformation of a process fuel stream such as methanol, kerosene, diesel and other alcohols or hydrocarbon-based fuels. In other systems, hydrogen storage devices, such as metal hydrides or high pressure gas cylinders are used to store hydrogen fuel on-board, as well as or instead of an on-board reformer.
To be effectively employed in submarine applications, fuel processing components such as vaporizers, reformers and hydrogen separators should be compact, robust and reliable. Examples of radial flow fuel vaporizer and reformer designs are disclosed in U.S. Pat. No. 5,676,911 which is incorporated herein by reference in its entirety.
Diffusion membrane hydrogen separators, such as palladium or palladium alloy membrane separators, are particularly suitable for use in reformate stream purification on a submarine since they are compact and their operation is not affected by the motion of the vessel. They generally separate the reformate stream into a high purity hydrogen stream and a raffinate stream, which is depleted in hydrogen and contains other reformate stream components, primarily carbon dioxide. The substantially pure hydrogen stream is then delivered to the fuel cell stack as the fuel stream.
A conventional palladium alloy-based hydrogen separation device includes an arrangement of thin-walled palladium alloy tubular members sealed in a shell (similar to a conventional shell-and-tube heat exchanger). In operation, a hydrogen-containing reformate gas stream is fed to one side of the tubular member. The hydrogen selectively diffuses through the palladium alloy material, thereby creating a stream of substantially pure hydrogen gas on one side of the tubular member and a raffinate stream on the other side. While these shell-and-tube separators are useful in providing a substantially pure hydrogen stream, such separators tend to be bulky and costly. In addition, the interface between the palladium alloy tubular members and the separator shell are prone to leakage and other breaches, particularly at high differential pressures.
A variety of non-conventional approaches to metal diffusion membrane separator design may be used as alternatives to the shell-and-tube configuration mentioned above. These include composite noble metal tubes, thin film metal deposition on porous substrates, or supported rolled film designs. In these designs the metal or metal alloy film thickness is drastically reduced compared to conventional palladium alloy tubes, reducing the cost of the unit for a given capacity of gas separation. Structural strength in these designs is provided by a less expensive hydrogen permeable support material, while the thin, supported metal or metal alloy film provides hydrogen selectivity. Examples of such designs are described in Edlund et al. U.S. Pat. No. 5,645,626 and related patents, and in Buxbaum U.S. Pat. Nos. 5,108,724 and 5,215,729.
Selection and control of operating temperatures and pressures in the various subsystems and components of the fuel cell power plant are important aspects of submarine fuel cell power plant design. In particular, control of the temperature in the catalytic reformer is important if efficient conversion without catalyst damage is to be achieved. Direct heating of the reformer by combustion gases may lead to hot spots in the reformer and sintering of the catalyst. In the present approach, a catalytic burner is used to heat a heat transfer fluid, which in turn is used to heat the vaporizer and reformer. The heat transfer fluid provides a thermal buffer between the catalytic burner gases and the vaporizer and reformer, and may be used to deliver large amounts of heat over a narrow temperature range, reducing the risk of catalyst damage. Relative to conventional flame burners, catalytic burners are generally safer, and result in more complete combustion of the reactants. Further, because they operate at a lower reactant gas concentration, the temperature is more readily controlled.
In selecting preferred operating pressures for the fuel cells and for the reactant supply and processing systems, factors such as the need to be able to discharge waste exhaust streams overboard at diving depth pressure should be considered. One approach is to operate the entire fuel cell power generating system, including the fuel processing system and fuel cells, at a pressure higher than the typical maximum depth pressure so that waste exhaust streams may be discharged without the need for further pumping. This is the preferred option when the fuel cell is operating on a dilute or impure fuel stream, and there is not a closed fuel loop for recirculating the fuel stream through the fuel cell stack. In this case the fuel stream exiting the stack must be vented or discharged from the stack, and ultimately from the vessel, so preferably it is at high pressure.
If the subsystems generating waste exhaust streams are operating at lower pressures than depth pressure, the exhaust gases must be compressed prior to discharge to the ocean, be it directly or via a scrubber. Compressing exhaust streams for discharge overboard represents a significant parasitic power load and requires the use, storage and maintenance of additional equipment.
In the present approach, the fuel cell is operated at low pressure (for example, below 100 psia) on a substantially pure fuel stream, and the vaporizer and reformer are operable at high pressure (higher than typical depth pressures, for example, 400-600 psia). It is preferable to operate the fuel cell at lower pressure for reduced system complexity and improved safety. The use of a substantially pure fuel stream means that the fuel stream can be recirculated through the fuel cell stack in a closed-loop system. Therefore in normal operation there is no fuel exhaust stream from the stack to be discharged overboard so low pressure operation of the stack is not a problem. The high operating pressure capability of the fuel processing system means that the exhaust gas therefrom is generally at a sufficiently high pressure for discharge directly overboard or via a scrubber, without the need to increase the pressure.
This approach requires the hydrogen separation membrane to be operable with a high differential (transmembrane) pressure across it, such that on one side it is fed with a reformate stream at a pressure approximately equal to the operating pressure of the fuel processor, and the hydrogen stream on the other side is at the preferred fuel stream inlet pressure for the fuel cell.